The wound healing process involves participation of white blood cells, also known as leukocytes. Leukocytes include granulocytes, monocytes and lymphocytes. Among the different types of granulocytes are neutrophils, basophils and eosinophils. Monocytes differentiate into macrophages, which are responsible for engulfment of tissue debris or invading foreign substances. Three common types of lymphocytes are T-cells, B-cells and natural killer cells. T-cells and B-cells play important roles in the recognition of antigens in the body (Parkin, 2001). Natural killer (NK) cells identify infected cells by alterations in the levels of the major histocompatability complex (MHC), and destroy the infected cells (Moretta, 2008).
The process of wound healing occurs in three overlapping phases. (Li, 2007; Broughton, 2006; Tsirogianni, 2006; Singer, 1999; Martin, 1997). The first phase is the inflammatory phase. It is characterized by recruitment of neutrophils, followed by monocytes to the wound site, where they kill and phagocytize bacteria (Agaiby, 1999).
The second wound healing phase which is known as the proliferative phase, involves formation of new granulation tissue. Fibroblasts proliferate and migrate into the wound space and synthesize collagen and other components of extracellular matrix (Greiling, 1997). At the same time, angiogenesis occurs, providing nutrients and oxygen to the metabolically active new granulation tissue (Tonnesen, 2000). Keratinocytes from the intact epidermis start to migrate over the provisional matrix and begin to proliferate, leading the way for new epithelial tissue (Kim, 1992).
Remodeling is the third and final phase in wound healing. It is characterized by fibroblast differentiation into myofibroblasts, which contract and bring the wound edges closer together (Tomasek, 2002). Remodeling of the collagen fibers by degradation and re-synthesis allows the wound to gain strength by re-orientation of the collagen fibers (a process tightly controlled by growth factors) (Werner, 2003).
The participation of leukocytes in the healing process is largely associated with their production of cytokines and growth factors (Keen, 2008). Recruitment of leukocyte subsets is regulated by chemoattractant cytokines (chemokines) such as interleukin (IL)-8, growth-regulated protein alpha (GRO-a), and monocyte chemotactic protein-1 (MCP-1) that activate and selectively guide various leukocyte subsets to wounds (Rossi and Zlotnik, 2000). Granulocytes produce proteases and reactive oxygen intermediates Macrophages produce proinflammatory cytokines and growth factors (Riches, 1996). Proinflammatory cytokines, including interleukins 1-alpha and 1-beta (IL-1-alpha and IL-1-beta), IL-6, and tumor necrosis factor (TNF)-alpha, play an important role both at the inflammatory and proliferative phase of wound repair. These cytokines also regulate the immune response. Angiogenesis is potentiated by hypoxia, nitric oxide (NO), VEGF and fibroblast growth factor 2 (FGF-2) (Liekens, 2001). TGF-beta contributes to the fibrotic process by recruiting fibroblasts and stimulating their synthesis of collagens I, III, and V, proteoglycans, fibronectin and other ECM components. TGF-beta concurrently inhibits proteases (C. U. Niesler, and M. W. J. Ferguson, 2001). PDGF, which is released by platelets and also by activated macrophages, endothelial cells and fibroblasts, is a major player in regulating fibroblast and smooth muscle cell recruitment and proliferation (C. H. Heldin and B. Westermark, 1999). Similarly lymphocytes, besides being immunological effector cells, also produce growth factors (Blotnik, 1994), and contribute to tissue remodeling during the late phase of wound healing.
The challenge of treating wounds is often compounded by patients with multiple pathologies such as diabetes, coronary artery disease and hypertension. These diseases have the common effect of exacerbating vascular complications due to various physiological conditions. Complications from wounds may result in increased morbidity and mortality (Doshi, 2008).
Conventional wound treatments include surgical debridement, antibiotic therapies and various dressings (Moran, 2008; Fonder, 2008). Wounds resistant to conventional treatment are also referred to as refractory wounds. These wounds lead to a decrease in quality of life and can result in increased morbidity and mortality. Thus, a need continues to exist for effective compositions and methods for wound healing.